Time and room: 17:15, lecture hall IAPAbstract: The laws of quantum mechanics allow for quantum cryptography, i.e. the distribution of a secret random key between two parties. When generalising this idea to the situation where more than two parties want to establish a common secret key, one can use certain multipartite entangled states as a resource. In the security analysis for this multi-user scheme some intricate new features arise and will be discussed. Finally, it is shown that our protocol for multipartite quantum cryptography offers a speed-up in certain quantum networks with bottlenecks.

Time and room:13:15, lecture hall IAPAbstract: Nitrogen vacancy (NV) centers are promising candidates for quantum computation, with room temperature optical spin read-out and initialization, microwave manipulability, and weak coupling to the environment resulting in long spin coherence times. The major
outstanding challenge involves engineering coherent interactions between the spin states of spatially separated NV centers. To address this challenge, we are working towards the experimental realization of mechanical spin transducers.
The spin transducer consists of a magnetic mechanical resonator in proximity of the NV centers. Consequently, the magnetic field at the NV location depends on the resonator motion. On the other hand, spin flips of the electronic spin of the NV center exert a force on the resonator. Hence, the spin-resonator interaction can be used to mediate an effective spin-spin interaction between two distant NV centers that are coupled to the same mechanical mode. This principle is in close analogy to trapped ions that interact via a common mechanical mode and which have already demonstrated high fidelity quantum gates. To maximize the coherent spin-resonator coupling it is required to employ a low
mass, high quality mechanical resonator, NV centers with very long spin coherence times, strong magnetic field gradients, and to combine them while preserving the excellent properties of the individual components. To date, we have successfully fabricated doubly-clamped
silicon nitride mechanical resonators and fabricated nano-magnets on top of them while maintaining a high-quality factor (Q>105). In addition, the resonators are integrated close to a bulk diamond sample to access bulk NV centers with long coherence times and to maximize the spin resonator coupling. In a second approach, we start with a levitated micromagnet and aim at using its degrees of freedom to couple to the NV-center spin. The absence of any support structure gives a large magnetic moment to mass ratio, which is favorable for large couplings, and can give rise to low mechanical damping.
In this talk, I report on our experimental progress towards achieving a coherent coupling of the motion of these resonators with the electronic spin states of individual NV centers under cryogenic conditions. Such a system is expected to provide a scalable platform for mediating effective interactions between isolated spin qubits and to enable the preparation of non-classical states of motion of a macroscopic object.

Time and room: 17:15, lecture hall IAPAbstract: We report on the production and study of a mixture of Bose and Fermi superfluids.
Such a mixture has long been sought in liquid helium where superfluidity was achieved separately in bosonic 4He and fermionic 3He. However due to strong interactions between isotopes, phase separation occurs when the 3He concentration exceeds 6%, which, so far, has prevented reaching simultaneous superfluidity for both species.
Using dilute quantum gases where interactions can be tuned, we have produced a Bose-Fermi mixture where both species are superfluid [1]. By exciting center of mass oscillations of the mixture we probe the collective dynamics of the system. Coherent energy exchange between the Bose and Fermi gas is observed with very small damping below a certain critical velocity. We compare this critical velocity for superfluid counterflow to a recent theoretical prediction [2,3]. Raising the temperature of the system slightly above the superfluid transition reveals an unexpected phase-locking of the oscillations induced by dissipation. Finally the lifetime of the Bose-Fermi mixture is governed by a very simple formula involving the fermionic two-body contact [4].

Time and room: 17:15, lecture hall IAPAbstract: During the past years, progress in controlling many-body quantum systems has led to the emergence of a new bottle neck: The number of parameters required to fully characterize a system has reached a regime where naive methods are no longer applicable. This is problematic in the emergent field of quantum technologies, where the verification and characterization of components is an important objective. The development is in no way unique to quantum physics. Indeed, classically, the theory of ill-defined high-dimensional estimation problems has become a successful focus of mathematical data science. I'll report in particular on work we have done in the context of "compressed sensing", which bridges the classical and the quantum theories.